Vulcanized elastomeric articles are commonly produced by transfer, compression, and injection molding. Each of these processes has advantages and drawbacks and fills a particular segment of the market. Injection molding has the advantages of high output, consistency, and ease of automation. Therefore, despite the need for higher capital investment it is the fastest growing segment.
Injection molding of elastomeric articles is presently carried out by feeding an injection molding press with a continuous strip or pellets of premixed elastomeric compound. The compound generally includes a base elastomer, fillers, plasticizers, process aids, anti-degradants, activators, accelerators, colors and cross linking agents. The elastomer is premixed with these additional components to form a homogeneous mass.
In order to obtain the uniform homogenous mass the elastomer and additives including the vulcanizing agent and accelerator are typically premixed in a high shear internal mixer such as a Banbury or a two roll mill. This high shear mixing action increases the temperature of this elastomer formulation, developing a heat and shear history in the elastomer, and, in some cases, may initiate partial cross-linking.
Another conventional technique for premixing or compounding elastomers is a two pass mixing cycle utilizing a mixer such as a Banbury mixer. In the first cycle the bulk of the components, with the exception of the cross linking agent itself, is incorporated into a homogeneous base compound in a high shear mixer. The heat generated during this first cycle is so great that the addition of cross linking agents would initiate the cross linking and result in a scorched composition. In the second stage of the technique the cross linking agent is added to the product of the first cycle at a lower temperature. This method reduces the scorch problem but nevertheless creates a heat history in the elastomer. The presence of the cross-linking agent and accelerator in the final blend still decreases its shelf life since some vulcanization or cross-linking occurs even at room temperature.
It has been proposed to maintain the elastomer separate from the curing agent with the curing agent maintained or encased in a non-cross-linkable polymer. This would tend to avoid premature crosslinking. One such attempt is disclosed in Callan U.S. Pat. No. 3,976,530. Callan discloses forming a two component elastomeric tape wherein one component is a first polymer having a vulcanizing agent incorporated therein and wherein the polymer is substantially non-cross-linkable by the vulcanizing agent. The second component of the polymeric tape is a second polymer curable by the vulcanizing agent contained by this first polymer. The two components of the tape are joined side by side. Such methods are expensive and often require extensive milling to provide a homogenous dispersion.
Abbott U.S. Pat. No. 3,941,767 and an article entitled Injection Molding of Conventional Formulations Based on Starch Encased Powder Rubber in the Journal of Elastomers and Plastics, Vol. 7, April, 1975, p. 93, report injection molding of starch encased powder elastomers. These references disclose that a starch encased elastomer can be fed into an injection molding machine and require less preliminary mixing. These starch encased powders however have limited application because polysaccharides are poor rubber additives having very bad water and chemical resistances compared to standard reinforcements such as carbon black, silica, clay, talc, etc. Further the article particularly points out that a special masticating feed screw is required.
Buchanan et al U.S. Pat. No. 3,673,136 discloses that a dry polysaccharide elastomer latex coprecipitate can be ground to form a free flowing fine powder. A powdered blend can be formed by blending powder curatives with the powder elastomer to give storage stable powder rubber compounds. The powdered compounds reportedly can be directly molded into rubber articles in ordinary heated compression molds or formed into finished rubber articles by extrusion from a simple machine with no prior shear mixing and also by injection molding. Again, polysaccharide reinforcing agents are required which are unsuitable for general use applications and require modification of existing polymer formulation.
In summary, commonly used methods of forming elastomeric compounds create a heat history in the compound. These methods also limit the shelf life of the compounds. Other known methods require the use of special modified polymers to provide a homogeneous compound. Since modified elastomers are required, these methods have only limited utility.